1. Field of the Invention
The present invention relates generally to chemical mechanical polishing (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to a substrate carrier having an active sacrificial retaining ring.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform CMP operations, including polishing, buffing and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. At each metallization level and/or associated dielectric layer, there is a need to planarize the metal and/or dielectric material. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization.
In the prior art, CMP systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to polish, buff, and scrub one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
In a typical CMP system, a wafer is mounted on a carrier, which rotates in a direction of rotation. The CMP process is achieved when the exposed surface of the rotating wafer is applied with force against a polishing pad, which moves or rotates in a polishing pad direction. Some CMP processes require that a significant force be used at the time the rotating wafer is being polished by the polishing pad.
Normally, the polishing pads used in the CMP systems are composed of porous or fibrous materials. Depending on the type of the polishing pad used, slurry composed of an aqueous solution containing different types of dispersed abrasive particles such as SiO2 and/or Al2O3 may be applied to the polishing pad, thereby creating an abrasive chemical solution between the polishing pad and the wafer.
FIG. 1A depicts a cross-sectional view of an exemplary prior art CMP system. The CMP system of FIG. 1A depicts a carrier head 100 engaging a wafer 102 utilizing a retaining ring 101. The carrier head 100 is applied against the polishing pad surface 103a of a polishing pad 103 with a force F. As shown, the top surface of the retaining ring 101 is positioned above the front surface of the wafer 102. Thus, while the front surface of the wafer 102 is in contact with the polishing pad surface 103a, the surface of the retaining ring 101 is configured not to come into contact with the polishing pad surface 103a. 
Several problems may be encountered while using a typical prior art CMP system. One recurring problem is called xe2x80x9cedge-effectxe2x80x9d caused by the CMP system polishing the edge of the wafer 102 at a different rate than other regions, thereby creating a non-uniform profile on the surface of the wafer 102. The problems associated with edge-effect can be divided into two distinct categories of the xe2x80x9cpad rebound effectxe2x80x9d and xe2x80x9cedge burn-off effect.xe2x80x9d FIG. 1B is an enlarged illustration of the pad rebound effect associated with the prior art. The pad rebound effect occurs when the polishing pad surface 103a initially comes into contact with the edge of the wafer 102 causing the polishing pad surface 103 to bounce off the wafer 102. As the moving polishing pad surface 103a shifts under the surface of the wafer 102, the edge of the wafer 102 cuts into the polishing pad 103 at the edge contact zone 104c, causing the polishing pad 103a to bounce off the wafer 102, thereby creating a wave on the polishing pad 103.
Ideally, the polishing pad 103 is configured to be applied to the wafer 102 at a specific uniform pressure. However, the waves created on the polishing pad 103 create a series of low-pressure regions such as an edge non-contact zone 104a and a non-contact zone 104a, wherein the removal rate is lower than the average removal rate. Thus, the regions of the wafer 102 which came into contact with the polishing pad surface 103a such as the edge contact zone 104c and a contact zone 104b, are polished more than the other regions. As a result, the CMP processed wafer will tend to show a non-uniform profile.
Further illustrated in FIG. 1B is the edge xe2x80x9cburn-off.xe2x80x9d As the polishing pad surface 103a comes into contact with the sharper edge of the wafer 102 at the edge contact zone 104c, the edge of the wafer 102 cuts into the polishing pad 103, thereby creating an area defined as a xe2x80x9chot spot,xe2x80x9d wherein the pressure exerted by the polishing pad 103 is higher than the average polishing pressure. Thus, the polishing pad surface 103a excessively polishes the edge of the wafer 102 and the area around the edge contact zone 104 (i.e., the hot spots). The excessive polishing of the edge of the wafer 102 occurs because a considerable amount of pressure is exerted on the edge of the wafer 102 as a result of the polishing pad surface 103a applying pressure on a small contact area defined as the edge contact zone 104c. As a consequence of the burn-off effect, a substantially high removal rate is exhibited at the area within about 1 millimeter to about 3 millimeters of the edge of the wafer 102. Moreover, depending on the polisher and the hardware construction, a substantially low removal rate is detected within the edge non-contact zone 104axe2x80x2, an area between about 3 millimeters to about 20 millimeters of the edge of the wafer 102. Accordingly, as a cumulative result of the edge-effects, an area of about 1 millimeter to about 20 millimeters of the edge of the resulting post CMP wafers sometimes could be rendered unusable, thereby wasting silicon device area.
Although, occasionally, an air bearing has been implemented in an attempt to compensate for the different levels of pressure applied by the polishing pad 103, air bearings have almost never been able to completely compensate for the difference in the pressure levels. Particularly, at the edge contact zone 104c, the edge non-contact zone 104axe2x80x2, the contact zone 104b, and the non-contact zone 104a the use of air bearings do not completely compensate for the difference in the exerted pressure, as the air can easily escape.
A common problem associated with the pad rebound effect and the edge burn off effect is the non-uniformity of the wafer 102 caused by the lack of uniform distribution of slurry between the polishing pad surface 103a and the surface of the wafer 102. As the edge of the wafer 102 cuts into the polishing pad surface 103a, it causes the slurry to be squeezed out of the polishing pad 103, thereby preventing the polishing pad surface 103a from performing a thorough polishing operation on the edge of the wafer 102. Thus, to accomplish a proper polishing operation, additional slurry must be supplied to the polishing interface. Consequently, a significant amount of slurry is wasted as a result of the combined effects of the pad rebound effect and edge burn-off effect.
In view of the foregoing, a need therefore exists in the art for a chemical mechanical polishing system that substantially eliminates damaging edge-effects and their associated removal rate non-uniformities while efficiently facilitates slurry distribution.
Broadly speaking, the present invention fills these needs by providing a system, which yields a substantially uniform removal rate throughout the surface of a wafer. In a preferred embodiment, the CMP system is designed to implement an active retaining ring configured to have a sacrificial component, which simulates the pattern of the substrate being polished by utilizing a plurality of collimated holes. As the sacrificial component is being polished together with the wafer, the edge of the polishing interface is thus virtually extended to the outside of the substrate being polished, thereby eliminating the aforementioned edge-effects, pad rebound effects, and edge bum-off effects. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a retaining ring structure of a carrier head for use in a chemical mechanical polishing system (CMP) is disclosed. The retaining ring structure includes a retaining ring support and a sacrificial retaining ring designed to confine a substrate to be polished. The sacrificial retaining ring also has an upper surface and a contact surface. The upper surface of the sacrificial retaining ring is configured to be attached to the retaining ring support, such that the retaining ring support holds the sacrificial retaining ring. The contact surface of the sacrificial retaining ring is configured to be substantially planer with a top surface of the substrate being polished.
In another embodiment, a wafer holding and application apparatus for use in chemical mechanical polishing (CMP) applications is disclosed. The apparatus includes a carrier head and a retaining ring support, which is designed to be attached to the carrier head. Also included in the apparatus is a sacrificial retaining ring, which is attached to the retaining ring support. The sacrificial retaining ring is designed to confine a wafer at a desired location when the carrier head applies the wafer to a polishing surface. The retaining ring support is defined from a material that approximates the wafer. A contact surface of the sacrificial retaining ring is positioned approximately planar with a to be polished surface of the wafer.
In yet another embodiment, a method for making a carrier head to be used in chemical mechanical polishing (CMP) of a wafer is disclosed. The method includes generating a retaining ring support and attaching the retaining ring support to the carrier head. Also included in the method is generating a plurality of capillary tube array units each having a contact surface. The method further includes attaching each of the plurality of capillary tube array units around the retaining ring support such that the plurality of capillary tube array units define a sacrificial retaining ring designed to contain the wafer having a surface to be polished. In addition, the surface of the wafer to be polished and the contact surface of each of the plurality of capillary tube array units are defined at about a same planar position.
The advantages of the present invention are numerous. Primarily, in contrast to prior art CMP systems, the contact surface of the sacrificial retaining ring is positioned substantially on a same horizontal plane as the top surface of the wafer, thereby virtually extending the polishing interface to the outside of the surface of the wafer. As such, the present invention eliminates the negative effects of the edge-effects, pad rebound effects, and edge burn-off effect. In addition, the construction of the sacrificial retaining ring out of plurality of capillary tube array units having plurality of capillary tubes facilitates the uniform distribution of slurry to the polishing interface so as to achieve a substantially uniform material removal through out the surface of the wafer.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.